1. Field of the Invention
The present invention relates to an LED adapted to allow light from an LED chip and excitation light from a fluorescent material to be emitted as color mixture light.
2. Description of the Related Art
A white LED is hitherto known that converts, through its transparent resin portion, blue light emitted from a blue LED chip into yellow light for mixture with the blue light to externally emit a resultant white light.
Such a white LED is configured as shown in FIG. 3, for example. Referring to FIG. 3, the white LED 1 includes a chip substrate 2, a blue LED chip 3 mounted on the chip substrate 2, and a transparent resin portion 4 having a fluorescent material 4a mixed therein. The transparent resin portion 4 is formed on top of the chip substrate 2 in such a manner as to surround the blue LED chip 3.
The chip substrate 2 is formed from a heat-resistant resin, such as a flat copper-clad wiring board for example, and includes on its surfaces a chip mount land 2a, a connection land 2b, and surface mounting terminals 2c and 2d extending therefrom via opposite end edges to its undersurfaces. The blue LED chip 3 is joined via e.g., die bonding, onto the chip mount land 2a of the chip substrate 2. An electrode on top of the blue LED chip 3 is electrically connected via wire bonding to the adjoining connection land 2b. The blue LED chip 3 is formed as a GaN chip, for example, such that it emits light having a peak wavelength at 450 to 470 nm, for example, when a drive voltage is applied.
The transparent resin portion 4 is formed from, e.g., transparent epoxy resin with the fluorescent material 4a in the form of fine particles mixed therein, and is molded onto the chip substrate 2 before hardening. During operation, blue light from the blue LED chip 3 impinges on the transparent resin portion 4 so that the fluorescent material 4a is excited to generate yellow light as excitation light from the fluorescent material 4a, thus emitting white light as a result of mixture of the different colors of light.
The fluorescent material 4a is, for example, a fluorescent material issuing a wide range of colored lights around true yellow. The fluorescent material is a material such as cerium-doped YAG fluorescent material, cerium-doped TAG fluorescent material, or orthosilicate fluorescent material (BaSrCa)SiO4. The fluorescent material 4a being arranged to issue a fluorescent light having its peak wavelength at 530 to 590 nm, for example.
According to the thus configured white LED 1, application of the drive voltage to the blue LED chip 3 via the surface mounting terminals 2c and 2d allows emission of light from the blue LED chip 3. The resultant blue light impinges on the fluorescent material 4a mixed in the transparent resin portion 4 to thereby excite the fluorescent material 4a to generate yellow light. This yellow light mixes with the blue light from the blue LED chip 3, resulting in white light issued to the exterior.
However, as size-reduction requirements progress in the thus configured white LED 1, there arises a need to secure the space for a bonding wire from the blue LED chip 3 to the connection land 2b, making it difficult to position the blue LED chip 3 at or near the center of the chip substrate 2 or near the center of the transparent resin.
As shown in FIG. 3, this leads to a comparatively elongated distance by which the blue light L1 issued from the blue LED chip 3 toward the connection land 2b travels through the interior of the transparent resin portion 4. Consequently, more fluorescent material 4a is excited by the blue light L1 from the blue LED chip 3, allowing the light externally emitted from the transparent resin portion 4 in the direction of the blue light L1 to contain more yellow light which can result in a yellowish white light.
Furthermore, since the transparent resin portion 4 is made, for example, by molding and then hardening epoxy resin with the fluorescent material 4a mixed therein, the fluorescent material 4a tends to settle onto the underlying chip substrate 2 before the hardening, as shown in FIG. 4. This characteristic is due to gravity, and based on the difference in the specific gravity between the fluorescent material 4a and the epoxy resin.
For this reason, light L2 outgoing laterally from the sidewalls of the blue LED chip 3 travels through part of the transparent resin portion 4 containing less fluorescent material 4a. This results in less fluorescent material 4a being excited by the blue light L2 from the blue LED chip 3, whereupon the outgoing light from the transparent resin portion 4 to the exterior in the direction of the blue light L2 contains less yellow light which can result in white light that is short of yellow tint (having a blue tint).
This means that different colors of light appear depending on the viewing direction in such a type of LED.